When was my IPad purchased?

The MUUL2B/A model number corresponds to a 2019 iPad Air (3rd Generation). When it was purchased is not important. It is when it was released by Apple. And yes, the 3rd Generation iPad Air was released by Apple in 2019.

So, yes, any cases meant for a 2019 iPad Air or an iPad Air 3rd Generation will fit.

Your sales invoice will provide a purchase date. If you don't have your original sales invoice, you might be able to estimate the purchase date from your device warranty:

Check Your Service and Support Coverage - Apple Support

Different iPad models have different physical dimensions. If you have a third generation iPad Air (a.k.a. iPad Air3) as you suggest, you will need to seek a case/cover that is explicitly designed for this model. Cases designed for the iPad Air4 will not fit an iPad Air3.

Being an older model, Apple no longer market fitted cases/covers for the iPad Air3. As such, you'll need to consider cases from third-party manufacturers - for example STM:

https://www.stmgoods.com.au/product/dux-plus-duo-ipad-air-3rd-gen/